Conversion treatments which provide the surface of aluminum with an excellent corrosion resistance and paint adherence are typified by chromic acid-based chromating treatments and phosphoric acid-based chromating treatments. The conversion coatings produced by chromic acid-based chromating treatments (Japanese Patent Publication Number 45-41088 [41,088/70] and British Patent Number 1,087,757) contain hexavalent chromium and therefore have a yellow or gold color. Thus, the color of this film will remain visible when a clear coat is applied on the surface. Moreover, since the conversion coating contains toxic hexavalent chromium, the development of a Cr(VI)-free conversion coating is desired.
In contrast to the preceding, the coatings formed by phosphoric acid-based chromating treatments do not contain hexavalent chromium and thus are colorless. As a result, the use of this treatment has been spreading in recent years. The phosphoric acid-based chromating treatment disclosed in U.S. Pat. No. 2,438,877 employs a treatment bath of chromic acid (CrO.sub.3), orthophosphoric acid (H.sub.3 PO.sub.4), and hydrofluoric acid (HF), and the principal component of the resulting film is chromic phosphate hydrate (CrPO.sub.4 .multidot.4H.sub.2 O). When this particular phosphoric acid-based chromating treatment is run in an aluminum coil line using the older conversion treatment and times of 5 to 10 seconds, a strongly corrosion-resistant, highly paint-adherent conversion coating having a chromium add-on of 20 to 50 mg/m.sup.2 is produced thereby.
Also within the sphere of Cr(VI)-free conversion treatment baths, Japanese Patent Application Laid Open [Kokai or Unexamined] Number 63-86875 [86,875/88] teaches the addition of molybdate to a zinc phosphate conversion treatment bath. However, the purpose of this conversion treatment bath is to passivate the surface of composite materials fabricated from different types of metals, and it also contains trivalent chromium, zinc oxide, and acetate as essential components in addition to molybdate.